1. Field of the Invention
The invention relates to a dispenser for a liquid, in particular to a bottle-top dispenser, with a cylinder/piston arrangement having a measuring cylinder and a piston which is adapted to be displaced therein in a sealed manner, with a valve arrangement connected to the piston, with an inlet valve and an outlet valve, each of which is provided with a valve body and a valve seat.
2. Description of Related Art
Dispensers are dispensing devices for dispensing defined volumes of a liquid. Familiar, in particular, are bottle-top dispensers and burettes. A bottle-top dispenser comes into operation, in particular, in order to issue an adjustable volume of liquid repeatedly from a bottle or flask, on the top of which the dispenser is attached.
Known bottle-top dispensers are designed for one-hand operation. In the case of dispensers with high suction power, for this purpose, a spring element which is designated as a suction spring is inserted between the cylinder/piston arrangement and the measuring cylinder thereof or an abutment connected thereto, so that the piston is raised under spring force in the measuring cylinder for the purpose of aspirating liquid out of the bottle. To eject the liquid via the outlet valve, the piston is moved in again by hand against the action of the suction spring and the suction spring is tensioned for the next suction stroke. Dispensers also exist, of course, in which no suction spring is provided, but in which operation takes place by hand only in both directions of operation. Finally, motor-driven dispensers also exist.
In addition to the cylinder/piston arrangement and the valve arrangement with inlet valve and outlet valve, a dispenser normally has various other components, in particular, a manipulation casing connected to the piston and/or a volume setting and/or a valve body comprising the valve seats of the valves and/or an outer casing of the valve arrangement and/or a suction line extending from the inlet valve into the bottle and/or an ejection line leading from the outlet valve to an outlet point. In this respect, reference may be made, for example, to a bottle-top dispenser of the assignee of the present application (published German Patent Application DE 197 02 773 A1 and corresponding U.S. Pat. No. 6,015,067; BRAND “seripettor” brochure, 9943 61, 21/0898, issued by BRAND GMBH+CO Fabrik für Laborgeräte).
Dispensers of such a type are known from various suppliers (e.g., “REPIPET” dispensers from Barnstead International, Product Data Sheet LT 388X 6/4/30/02; ProLiquid “Minidispenser”, Product Data Sheet 2000, produced by ProLiquid GmbH, 88662 Überlingen, Germany).
In the parts or regions touched by the liquid, the known dispensers considered previously already are predominantly made of plastic material—specifically, for example, of polypropylene (PP), but also of polytetrafluoroethylene (PTFE) for individual components (Barnstead “REPIPET” brochure, loc. cit.). However, the components of the valves always are made of an elastomeric material, in particular, ethylene-propylene elastomers.
It has already become known to manufacture the valves provided in dispensers from plastic materials made of polytetrafluoroethylene (PTFE) or similar materials or comprising such materials. These valves have a higher resistance to chemicals than elastomer valves and are significantly cheaper than valves with a glass/ceramic pairing.
Glass and ceramics generally contain metal oxides. The same applies to elastomers. Reagents that elute metal oxides, particularly certain acids, find application in particular in trace analysis. Therefore, chemically bonded metal oxides etc. are also released and are accordingly troublesome in trace analysis. Metal oxides also occur in dyestuffs, in reinforcing materials and in fillers for plastic materials and also in processing aids.